The invention relates to a screened therapy Chamber for ion therapy, the chamber being for shielding neutrons having an energy up to GeV, wherein the therapy Chamber is shielded at all but one side, which includes a labyrinth-like shielded access.
In Germany and other European countries, therapeutic medical accelerators for highly energetic ion radiation are under development [I]. For the design of such high energy ion accelerators for cancer therapy, however, there is a problem in that ion accelerators produce secondary radiation during slowing down of the ions in the accelerator structures, in biological and other targets, in particular in the patient when irradiated. The main component of the secondary radiation is neutron radiation. The primary beam is accelerated and transported to the target.
The following processes result in the production of neutrons forming secondary radiation:                beam losses by charge conversion,        beam losses by charge transfer,        beam losses by interaction with the residual gas in a partial vacuum,        losses during deflection and inflection procedures extraction and injection,        during slowing down of the ion beam in the tissue or another material.        
The unavoidable secondary radiation, that is, the neutron radiation must be shielded. The radiation levels generated as source radiation are substantial; they are at a level of up to Sv/h. The radiation level tolerable outside the radiation shield is at a μSv/h level, depending on the definition for the areas outside the radiation shield, for example, according to German radiation protection rules surveillance or control areas. Consequently, the neutron radiation dose must be reduced by about 6 orders of magnitude.
A heavy ion therapy unit in a hospital environment needs to comply with the requirements of the radiation protection rules, that is, areas adjacent to the therapy rooms are to be defined as surveillance areas, in which the 1-mSv-limit per year is met with negligible emissions from radioactive materials.
Conventional therapy installations wherein patients are irradiated with X-ray or gamma radiation are arranged in radiation bunkers in such a way that concrete walls shield the primary radiation such as the stray radiation in such a way that the surrounding areas do not have an increased radiation level.
Radiation therapy has been performed so far using radiation from gamma radiation sources such as 137-Cs, 60-Co, or X-radiation generated by electron accelerators. The radiation protection obtained by walls serves therefore for the shielding of gamma and X-radiation. For electron accelerators with high end energies of up to 50 MeV, the neutron radiation generated by the nuclear photo effect must be shielded. Authoritative control rules for the design of the neutron shields are the DIN norms (German Industry Norms) DIN 6847/part 2 [II], the publication of the NCRP (National Council on Radiation Protection and Measurements) and, for high energy particle accelerators in basic research, the reference book Landolt-Börnstein [III].
The general procedure for the shielding of neutron radiation is the use of hydrogen-containing substances such as water, concrete and water-containing minerals. Materials such as lead or iron used for the shielding of X- and gamma radiation are not particularly suitable to absorb or moderate neutrons. For direct neutron radiation in accordance with DIN 6847, part 2, [II], the following tenth-value layer thickness for the area of mechanical irradiation units are given for a limit of 1 mSv per year, with negligible emissions of radioactive materials:
MaterialWater paraffinConcreteIron, lidTenth value thickness10-15 cm16-25 cm42 cm
Because of insufficient absorption and moderation of neutrons with energies of up to 3 MeV, the effectiveness of metals is insufficient so that additional hydrogen-containing absorbers must be used.
The shielding effect H(d)/H0 of a wall with a thickness of d and a minimum thickness of (d>d0) is for a neutron beam with an energy En:
            H      ⁡              (        d        )              Ho    =            1              r        2              ·          exp      ⁡              (                  -                      d                          λ              (                                                E                  n                                ,                ϑ                                                    )            With the characteristic moderating constant 2, which depends on the energy En of the neutron radiation and the angle relative to the incident beam, the distance r to the source location the shield thickness d and the source strength H0, which depends on the primary beam and the target.
Generally, the shielding effect is higher with copper than with concrete except for neutrons with energies of 3 MeV or less. The shielding however should be such that it is effective for all neutron energies as they may occur with the transport of the source neutrons through the shield so that shielding must be present which is effective for all neutron energies.
The radiation protection arrangements of the radiation therapy installations built up to now concentrate in the shielding of neutron components in the energy range of the neutrons of about 10 MeV. Herein, concrete alone is generally a sufficiently effective radiation protection for all types of radiation. The difference in the shielding effectiveness of metals and concrete is small over a wide energy range that is an energy range of 3 to 30 MeV.
The newest radiation protection plans for the Italian ion therapy project TERA were done by Agosteo et al. [IV]. Herein the planning is based on carbon ions. For the design of the radiation protection features, neutron spectra are used as they are used also in the present case. Agosteo developed on the basis of the measured neutron spectrums and the transport of the neutrons—using the radiation transport program FLUKA [V]—with a simplified geometry, for example, a spherical geometry, a model which permits an estimation of the attenuation of the neutron radiation in such simplified arrangements. The model describes essentially the dose caused by direct radiation. increased doses to be expected as a result of stray radiation are difficult to estimate with such models.
Heavy ion therapy installations which provide depth therapy with carbon ions require accelerated ions with energies of up to about 400 MeV per nucleon. The neutron radiation generated during moderation of the ions in the tissue has energies of up to about 1000 MeV. Such high-energetic neutron radiation is difficult to shield particularly with conventional shielding materials. The attenuation length of neutrons with energies in excess of 100 MeV in normal concrete of the density of 2.3 g/cm3 is 45 to 52 cm. The tenth value thickness is about 100 cm. The physical parameters of an ion therapy installation differ substantially from those of a conventional X-ray irradiation installation. The primary beam including protons, carbon ions, oxygen ions is precisely guided from the generation during the acceleration up to the deposition in the tissue and is not much scattered like X-ray beams, but, during moderation, highly energetic neutrons are generated. For example, a carbon ion with an energy of 400 MeV per nucleon produces about 5 neutrons on average when being slowed down.
Another basic difference with regard to conventional X-ray therapy installations resides in the higher spatial requirements and the spatial disturbance of the beam generation up to the application of the beam in the patient. Therefore the shielding expenses already of the beam transport system are higher than with conventional installations. Furthermore, the access to the treatment rooms is more difficult since large areas around the therapy unit are occupied by the beam guide structure. Conventional shielding concepts for X-ray irradiation installations utilize mainly the shielding effects of concrete with attenuation lengths (concrete wall thicknesses) which are applicable for MeV neutron radiation.
Radiation protection shields have been developed so far only with the consideration of hydrogen-containing moderators for the utilization of the elastic scattering of neutrons on protons, whose effective cross-sections become smaller with increasing neutron energy.